A force sensor is applied, for example, to detect a grasping force of a robot hand that is used in lines for manufacturing products. Here, the grasping implies an operation of firmly holding a workpiece by a robot hand. An increase of accuracy in control of the grasping force is expected by detecting the grasping force of the robot hand, and by performing feedback control on the grasping force, which is applied to the workpiece, in accordance with detected information.
A capacitive force sensor is known as one of various types of force sensors for detecting contact loads. In a typical structure of the capacitive force sensor, a membrane having flexibility is movably disposed on a substrate with a support member (also called a spacer or a post in this specification) interposed between them. An electrode is disposed in each of the membrane and the substrate (in this specification, the electrode on the membrane side is called an upper electrode and the electrode on the substrate side is called a lower electrode), and the upper electrode is deformable together with the membrane. When a load is applied to the membrane upon contact with a solid body, for example, the membrane is caused to flex, and the distance between the upper and lower electrode is changed. The magnitude of the applied load is detected by detecting the distance between the upper and lower electrode in terms of capacitance, and by converting the detected capacitance to an electric signal. A structure unit constituted by the membrane, the upper and lower electrodes, and the spacer is called a cell.
Force sensors are grouped, from the viewpoint of function, into a type of measuring a load, a type of measuring load distribution, and a type of measuring the direction of a load (e.g., tensile direction, compression direction, or shearing direction). In any type, the shape or the structure of a sensor surface is constituted such that a deformable portion of a detection device is deformed selectively with respect to a load applied in a direction or a position to be detected. In a related-art force sensor, a unit device for detection (i.e., a detection unit device) is constituted by one cell (see PTL 1).
In a force sensor used to detect a grasping force of an assembly robot, a contact surface with respect to a workpiece functions as not only a sensor, but also as a mechanical member for grasping the workpiece. When the sensor surface is deformed due to a load applied to the force sensor, such deformation may be one of factors causing a shift of the position at which the workpiece is grasped. Accordingly, there is a demand for a force sensor in which the deformable portion in the sensor surface is deformed in a small amount due to the load.